Commonly and widely used disposable diapers comprise an absorbing material that absorbs excreta such as urine and the like, a topside material that covers one surface of the absorbing material and touches the wearer's skin, and a backside material that covers the absorbing material and prevents leakage of fluid excreta. The diaper is formed by bonding these materials. Such a disposable diaper has an elastic means provided to prevent excreta from leaking out beyond the waist or leg region and a closure means, such as a closure tape, which is used to fasten the diaper on the front and rear waist regions when wearing the diaper.
As such a backside material, a polyolefin-based porous film having breathability and water vapor permeability is known. However, when the diaper is put on incorrectly or when the closure tape is peeled off to check for urination while the diaper is being worn, backside materials formed using porous film with insufficient strength have a problem of breakage, making it necessary to change the diaper for a new one. In the field of diapers for adults, the problem of insufficient leakage prevention exists in addition to the insufficient strength of the diapers, and the strength and leakage prevention properties of the diapers have actually been increased at the cost of breathability and water vapor permeability.
Thus, as a means for obviating the aforementioned drawbacks, a porous film having excellent breathability and water vapor permeability as well as improved strength and leakage prevention is strongly desired.
Polypropylene occurs in crystal states such as α crystals and β crystals. Under general crystallization conditions, α crystals, which are the most stable, are mainly produced, but β crystals can be produced preferentially by employing specific crystallization conditions or adding a β-crystal nucleating agent. Also, β crystals are known to undergo a crystal transformation into stable α crystals when subjected to thermal and mechanical action, and recently several methods have been proposed for producing a porous film, especially a breathable polypropylene film having continuous through-pores, which utilize the crystal transformation that occurs in the course of stretching (Japanese Unexamined Patent Publications Nos. 118429/1995, 176352/1997, 255804/1997 and 100720/1994).
The process for forming continuous through-pores is not yet fully clarified. However, various processes have been hypothesized as follows. Formation of as many β crystals as possible in an unstretched web sheet before stretching and subsequent stretching of the sheet causes a crystal transformation from β crystals into α crystals. In the course of the stretching, the transformation from β crystals with low crystal density to α crystals with high crystal density causes volume shrinkage and leads to exfoliation at the crystal surface, whereby pores are formed in the initial stage of stretching. Another possibility is that pores are formed by exfoliation at the crystal surface resulting from the difference in stretchability between α crystals and β crystals before the transformation into α crystals. Moreover, another possibility suggested is that crystal fineness, orientation and re-arrangement may participate in the pore formation. It is considered that, as the stretching process progresses thereafter, the size of the pores becomes gradually larger and adjacent pores are connected to each other, resulting in the formation of continuous through-pores.
Each publication mentioned above suggests the necessity of forming as many β crystals as possible in an unstretched web sheet before stretching it as a condition for obtaining a breathable film. As an index of the β-crystal content or proportion of β-crystals in the unstretched web sheet, a K value determined by X-ray diffraction measurement is employed. That is, the higher the K value is, the easier it is to obtain a film with good breathability. Thus, for example, Japanese Unexamined Patent Publication No.255804/1997 recommends that the K value be not less than 0.7, and preferably 0.8 to 0.98. However, it is known that by incorporating a certain β-crystal nucleating agent, the recommended K values can be achieved relatively easily without employing special crystallization conditions, and moreover, as will be described below, even a high K value does not necessarily result in a production of a film with excellent breathability.
For example, Japanese Unexamined Patent Publication No.176352/1997 discloses that a single use of a β-crystal nucleating agent cannot produce a film with sufficient breathability in spite of a high K value, and proposes a method for removing the drawbacks, which comprises adding and mixing particles of other resins than polypropylene. However, it is difficult to uniformly disperse these resin particles in the polypropylene resin, and the uneven dispersion tends to cause the films to break in the stretching step, and the films produced by the method are not preferable from the standpoint of recycling.